Parkinson's disease is one of leading neurodegenerative disease that occurs by instable generation and secretion of dopamine (16). In patients with Parkinson's disease, there have been damages in dopaminergic neuron in the midbrain; pathological features, such as formation of Lewy bodies; mobility defects, such as bradykinesia, rest tremor, and rigidity; and non-motor symptoms, such as depression, dementia, and insomnia (17-19).
Parkinson's disease is a neurodegenerative disease found mostly in older generations. Statistically, Approximately 1% of people aged more than 55 and 3% in people aged more than 75 have been diagnosed with the disease (20). As the population of aged people increases, patients diagnosed with Parkinson's disease are ever growing in number. Globally, the population of patients with this disease has been projected to increase from 4.1 million in 2005 to 8.7 million by 2030 (21, 22).
The cause of Parkinson's disease has been unclear; however, previous studies reported that it's caused by both genetic and environmental factors in combination; especially, mutation of parkin gene has the highest prevalence among the various genetic factors that cause Parkinson's disease. Parkin gene has been first discovered in Japanese stock that has autosomal recessive juvenile Parkinsonism (ARJP) (23). Parkin gene mutation could be discovered from approximately 50% in early-onset hereditary Parkinson's disease and 18% in sporadic patients below the age of 50 (24).
Parkin is comprised of 465 amino acid sequences that have functions as E3-ligase in ubiquitin-proteasome system. Parkin protein functions to reduce the oxidative stress in the cell by removing damaged, oxidized, and/or irregularly structured protein inside the cell.
When Parkin mutation occurs, it loses its property as an E3-ligase; inclusion body and/or irregular proteins are accumulated inside the cell that lead to reduced secretion of dopamine and apoptosis of dopaminergic neuron (25). There has been a recent study pertaining to Parkinson's disease using the fruit flies that have shown decrease in motor function by the decrease in dopamine secretion. Dopamine secretion has decreased due to an inactivation of dopaminergic neuron in which the function of Parkin and PINK1 (PTEN-induced putative kinase 1) was revealed (26). Moreover, when Parkin was overexpressed in the fruit fly that did not express PINK1, Parkinson's disease-related symptoms caused by PINK1, such as mitochondrial dysfunction and degradation of dopaminergic neuron, were confirmed to be recovered (26-28). Based on these factors, Parkin protein may successfully act as a target protein-based agent to treat Parkinson's related diseases. It functions as a main enzyme in the ubiquitin-proteasome system to destroy inclusion body and suppress apoptosis of dopaminergic neuron by maintaining the function of mitochondria from oxidative stress.